Incorporate feedback from LI student crashlogs.

 - loaders test for whether file exists

 - load cmd catches syntax errors

 - fixed clearing push-time errors when retracting rule

Documented (and add test cases) for few newly discovered bugs in BC.

Recompile relevant rules after receiving `:- backchain` pragma.
Fixed Issue #66 - "stale backchaining memos when new rules are added"

GBC computation issues proper replacement updates in the case were there was an
old value (i.e. item was non-null).

When rules are recompiled their index stays the same.

Rules which fail to compile are reported as errors, much like rules which fail
to initialize. Rule recompilation is now one rule at a time rather than all in
one go.

Added item hash function.

Updated doctests to reflect all of these changes.
added test case for issue #66